The quest for increasing channel capacity and data rates within restrictions of limited resources (especially bandwidth limited by allocation and power limited by portable battery life) has led to the advancement of various technologies. One technology in which such advancement has been made is in techniques for modulation and channelization using domains based on phase, amplitude and frequency. One example of such advanced modulation techniques is orthogonal frequency division multiplex (OFDM), which has been successfully utilized in wireless local area network (WLAN) applications such as 802.11a and HiperLAN/2. In OFDM, the data are sent simultaneously over equally-spaced carrier frequencies using Fourier transform techniques for modulation and demodulation. By proper choice of frequencies in the Fourier transform conversion, OFDM can squeeze multiple modulated carriers into a prescribed band while substantially preserving orthogonality to eliminate inter-carrier interference. The resulting OFDM transmission can be made robust to multipath while still providing high data rates under varying channel conditions.
Another domain proposed for enhancing channel capacity is the use of multiple-input multiple-output (MIMO) antenna structures. In spatial multiplexing, the input data stream is split into a number of parallel streams and transmitted simultaneously. Despite its benefits, MIMO systems are not yet popular due to their inherent complexity and need for multiple antenna structures.
Finally, the polarization domain has been used in a particular manner in which two orthogonally polarized antennae are used to generate two corresponding orthogonal polarization states that are employed to improve data throughput. An example of this is in satellite communications, in which one antenna (and polarization state) is used to transmit a first set of data and an orthogonally polarized antenna is used to transmit a second set of data, thus doubling data throughput without increasing the bandwidth.
A need exists for technology that will further increase data throughput within systems that have limited resources.
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